Lever escapement for timepieces



Dec. 20, 1960 A. BEYNER ETAL 2,964,901

LEVER ESCAPEMENT FOR TIMEPIECES Filed Feb. 3, 1959 INVENTORS Andr Begng and min ATTORNEY Uflitsd tews. .Patsi o sur-Fontainemelon, Switzerland, assignors to Ebauches S.A., Neuchatel, Switzerland Filed Feb. 3, 1959, Ser. No. 790,861 Claims priority, application Switzerland Mar. 6, 1958 7 Claims. (Cl. 58121) The present invention relates to a lever escapement for timepiece, and more particularly to a magnetic device for drawing the lever into its end positions and. for holding it in these positions. The said device is particularly intended for electric timepieces having a driving balance, but it may also be used in mechanical watches or clocks having a conventional lever escape ment, in order to do away with the draft effect.

One of the applicants has already described in his co-pending application Serial No. 711,749, filed January 28, 1958, a device of this type, more particularly adapted for use in a small clock. The novel construction, forming the subject matter of the present invention, is rather adapted to be used in watches of small dimensions, for instance in wristlet-watches. Applicants have tried to reduce to a minimum the inertia of the several parts and more particularly the inertia of the lever. A construction such as described in the above-mentioned application makes the lever heavier. In addition, the shape itself of the magnetic shunt of the said construction displaces the center of gravity of the lever away from its axis of rotation.

According to the invention there is provided a permanent magnet having an air gap and fixed to the frame of the movement of the timepiece, and a moving armature made of magnetizable material, secured to the pallet arbor, the said magnet being arranged in such a manner and the shape of the moving armature being such, that when the lever is in its middle position, the reluctance of the magnetic circuit is at a minimum. As the armature swings toward either pole the reluctance between the armature and that pole decreases and the reluctance between the armature and the opposite pole increases, thus providing magnetic bias of the lever against banking pins, the whole being arranged in such a way that the lever is drawn from the middle position to either of the banking pins and is held against these pins through a small torque originating from the attraction exerted by either of the magnet poles on the moving armature.

The accompanying drawing shows, by way of example, one embodiment of the invention.

Fig. 1 is an inverted plan view of this embodiment.

Fig. 2 is a cross-sectional view taken along the line Il--Il in Fig. 1.

The balance wheel 2 is pivoted, on the one hand, in the pillar plate 1 and, on the other hand, in a bridge (not shown). The balance wheel 2 is rigidly connected with a moving armature 3 cooperating with pole pieces (not shown) of an electromagnet, in order to sustain the oscillations of the balance wheel 2. The balance staff 4 carries a large roller 5 and a small roller 6 of conventional type. In the large roller 5 is fixed an impulse pin 7 adapted to cooperate with the fork 8 of the lever 9, whereas the small roller 6, provided with a 2 notch 10,"cooperates with the safety pin 11 in the usual way- The lever'9, driven on to its arbor 12, is pivoted, on the one hand, in the pillar plate 1 and, on the other hand, in a pallet bridge 13.1ts movement of oscillation is mechanically'limited by banking pins 14 secured to the pillar plate'l. The pins 14 are' madeof a nonmagnetizable material, for instanceof brass. The base 15 of the banking pins 14, having a larger diameter than the latter, is received in a recess 16 of the pillar plate 1. In the recess 16 is also disposed a horseshoe permanent magnet 17 made from an alloy having a great remanence and a high coercitive force, such as the products TiconaY or Alnico (registered trade names). The two poles 18 of the magnet 17 are separated by an air gap. The middle portion of the magnet '17, remote from the poles 18, has a lug 19 bearing against the large portions '15 of the banking pins 14, so that the magnet 17 is thus correctly oriented on the pillar plate 1. The magnet 17 is secured to the pillar plate 1 by any suitable means, e.g. by caulking, driving, riveting or sticking. s

- To the pallet arbor 12 is fixed a moving armature 20 of soft iron, acting as a magnetic shunt. The armature 20 is held. at a certain distance from the lever v9 by means of a spacing piece '21 secured to the arbor 12. The moving armature 20 has the shape of a finger oriented in the direction opposite to that of the fork of the lever 9, so that it contributes to balance the lever assembly. By suitably dimensioning the armature 20, it is possible to obtain that the center of gravity of the assembly 9, 20 be practically situated on the axis of the lever arbor 12, this being, from a static and dynamic standpoint, particularly upon shocks, an appreciable advantage. The moving armature 20 is placed at a small distance from the magnet 17; in a horizontal projection, its free end has a width which is scarcely smaller than the width of the air gap between the poles 18 of the magnet 17. The dimensions and the shape of the magnet 17 and of the moving armature 20 are chosen-in such a manner that when the lever 9 is in its middle position (position illustrated in Fig. 1), the reluctance of the magnetic circuit of the magnet 17 will be at a minimum, this total reluctance increasing on each side of this middle position up to the stop positions of the lever 9 against the banking pins 14. Due to this fact, the lever 9 is drawn from the middle position to either of the banking pins 14 and is held against these pins by means of a small torque produced by the attraction of one or the other of the poles 18 of the magnet 17 on the moving armature 20.

An escape wheel 22 which is pivoted in the pillar plate 1 and in a bridge (not shown) of the movement frame, is rotated step by step, always in the same direction, by the alternative action of the pallets 23 of the lever 9. A mechanism of known type, not illustrated in the drawing, transmits the movement of the escape wheel 22 to the hands of the watch.

On normal operation of the Watch, the lever 9 is led by the impulse pin 7 of the large roller 5, and the function of the magnetic shunt 20 only serves to eliminate the friction of the safety pin 11 against the periphery of the small roller 6, which friction would be detrimental to the rating of the watch.

In order to avoid straying of lines of force of the magnetic field of the magnet 17 towards the balance wheel 2 carrying the armature 3 made of soft magnetic material, there is provided a magnetic screen 24. This screen 24 is placed at right angle to the pallet arbor 12, approximately at the same distance from the moving 3 armature 20 and the lever 9. It is secured to the pillar plate 1 by any suitable means, for instance by means of screws engaging the holes 25. The area and the thickness of the screen 24 are dimensioned in such a manner that no line of force can disturb the regulating member, so that a good isochronism of the latter may be obtained.

As already said, the parts 9, 21 and '20 are keyed on the pallet arbor 12. For the assembling, the magnetic screen 24 has an elongated opening 26' through which may pass the moving armature 20. It is, therefore, possible to put in place the magnetic screen 24 on the pillar plate 1 and to insert the arbor 12 provided with the armature 20 into its lower bearing and then to secure the pallet bridge 13. The screen 24has a second opening 27, of circular shape, for the passage of the spindle 28 of the escape wheel 22.

The device as described and illustrated has, therefore, clear advantages over the prior devices, namely among others:

(a) Relative reduction of the moment of inertia of the lever;

(b) Static and dynamic balancing of this lever;

(c) Elimination of the straying of lines of force by means of a stationary magnetic screen.

What We claim is:

1. A lever escapement for a timepiece, comprising a lever, a spindle for the lever, stationary banking pins on opposite sides of said lever providing stop positions, said lever being swingable on its spindle between said banking pins, a stationary permanent magnet provided with an air gap, and a moving armature made of magnetizable material and secured to the spindle of the lever, said magnet being arranged in such a way and the shape of the moving armature being such, that when the lever is in its middle position, the reluctance of the magnetic circuit of the magnet is minimum, said reluctance increasing on movement to each side of said middle position up to the stop position of the lever against said banking pins, whereby the lever is drawn from the middle position to one or the other of the banking pins and is heldagainst this pin through the force exerted by the magnet poles on said moving armature.

2. In a lever escapement-according to claim 1., a reg ulating member and a stationary screen made of a soft magnetic material, the area and the thickness of said screen being dimensioned in such manner that no line of force of the magnet field can disturb said regulating member.

3. In a lever escapement according to claim 2, said magnetic screen being arranged at right angles to the spindle of the lever and positioned between the moving armature and the lever.

4. In a lever escapement according to claim 1, the lever being provided with a fork and the moving armature shaped like a finger oriented in the direction opposite to that of said fork of the lever, so that it contributes to the balance of the lever.

5. In a lever escapement according to claim 1, the permanent magnet having the shape of a horseshoe and being provided with an orienting .lug bearing against the banking pins.

6. In a lever esoapement according to claim 1, the banking pins being made from a non-magnetizable material.

7. In a lever escapernent according to claim 2, said magnetic screen having an opening forming a pasage for the moving armature facilitating the assembling of the escapement.

References Cited in the file of this patent UNITED STATES PATENTS 2,669,089 Straumann Feb. 16, 1954 

